Method of producing electric heating elements



1955 A. J. HUCK ETAL METHOD OF PRODUCING ELECTRIC HEATING ELEMENTS FiledJuly 1, 1950 United States Patent METHOD OF PRODUCING ELECTRIC HEATINGELEMENTS Alfred J. Huck and John J. Kueser, St. Louis, Mo., as-

signors to Knapp-Monarch Company, St. Louis, Mo., a corporation ofDelaware Application July 1, 1950, Serial No. 171,712

4 Claims. c1. 29-1555) This invention relates to a tubular heatingelement of the extruded, flattened type and particularly to a processfor forming the same whereby an inexpensive heating element of highefficiency can be produced.

One object of the invention is to extrude a coil of resistance wire anda core therefor of argillous material, sheath the same, fire it forreducing the argillous material to ceramic and then increase thedielectric strength of the core and provide for good heating transferfrom the resistance wire to the sheath by a flattening process whichcompresses the ceramic tightly and causes it to fill the entire interiorof the sheath and physically reinforce the sheath so that less warpageis experienced as it heats.

A further object is to provide a heating element formed by a processwhich flattens a sheathed heating element after it is fired by greatpressure that tightly compresses the ceramic produced by the firing andprovides a heating element having a cooler wire operating temperatureand longer life.

With these and other objects in view, our invention consists in a methodof producing a flattened, extruded tubular heating element whereby theobjects contemplated are attained, as hereinafter more fully set forth,pointed out in our claims and illustrated in the accompanying drawings,wherein:

Figure l is a partial side elevation and partial sectional view of anextruded heating element being formed in accordance with Smith PatentsNo. 1,951,176 of March 13, 1934, and No. 1,987,915 of January 15, 1935.

Figure 2 is a partial side elevation and partial sectional view showingthe heating element and its core being inserted in a metallic sheath.

Figure 3 is an enlarged sectional view on the line 33 of Figure 2.

Figure 4 is a similar sectional view showing the sheathed heatingelement after it has been fired and flattened in accordance with ourpresent invention.

Figure 5 is a similar sectional view showing a different flattened shapefor the heating element; and

Figure 6 is a plan view of the sheathed heating element embodying ourinvention and showing an intermediate step in the process, to wit, theheating element formed to a desired or required shape before it isflattened.

On the accompanying drawing we have used the reference numeral 10 toindicate a hopper for argillous material 12 which hopper has a dischargenozzle 14. The hopper and nozzle may be part of a heating elementforming machine of the kind shown in the above identified Smith patents.As disclosed therein, the argillous material 12 is under pressure sothat it is extruded from the nozzle 14.

Depending within the hopper 10 is a guide tube 16 for a coil ofresistance wire 18, the lower end of which is filled with and imbeddedin a core 12a of the argillous material 12 as the material is extrudedfrom the nozzle 14. The extrusion pressure and movement also serve to(ll'gwlllle resistance coil 18 downwardly through the guide tu e Theresistance coil 18 as shown in Figure 1 and the core 12a are permittedto dry slightly, the argillous material 12 being moist and in a plasticcondition when extruded from the nozzle 14. Drying is permitted justlong enough to permit handling and subsequent insertion into a metallicsheath 20 as shown in Figure 2, the sheath being preferably straight andthus permitting free entry of the core and resistance wire, and theinside diameter "ice of the sheath being slightly greater than thediameter of the core to permit ready assembly. The parts are then inrelation to each other as illustrated in cross section in Figure 3, thespace 22 representing the diflerence in diameters and being somewhatexaggerated in this figure. The foregoing steps for forming andsheathing a heating element are old in the Smith patents.

According to our invention, the sheathed heating element is then firedor heated to dry the moisture out of the argillous material 12 andreduce it to a ceramic. The firing process shrinks the core 12a whichincreases the space 22 and this space considerably reduces theefliciency of the heating element in operation. However, an extrudedheating element of this type is inexpensive to produce as compared withheating elements of the type which imbed the resistance coil in powderedinsulating material tamped into a sheath along with the resistance coil.

We have found that the efliciency of the heating element as disclosed inFigure 3 can be greatly increased however by flattening the sheathedelement after it has been fired, the result being to tightly compressthe ceramic thus resulting in good heat transfer from the resistancewire to the ceramic and from the ceramic to the sheath. This results ina cooler operating temperature of the resistance wire thus providinglonger heating element life, and the dielectric strength is greatlyimproved as well as the physical strength of the entire sheathed elementwhich then warps less than when the flattening process is omitted. Weare aware that heretofore metallic sheaths containing powderedinsulating material tamped in around resistance wires have beenflattened but to our knowledge first firing an extruded, sheathedheating element and thereafter flattening thereof as here disclosed hasnot been done.

For this purpose, we find that the argillous material after firing mustnot be in the nature of porcelain, china or steatite but more of theorder of common clay like that used in building bricks so that it willcrumble somewhat during the flattening process. A fired material of thisnature acquires additional denseness of structure under extremepressures such as those between 2,000 and 10,000 p. s. i. produced byhigh pressures or high striking forces imparted to the dies in theflattening press. This results in holding the particles together whereasbefore flattening the bonding of the particles due to firing holds themtogether at a certain lesser density of structure. After flattening, thehigh dielectric characteristics of the fired ceramic are retained andconsiderably improved upon as we have determined by numerous testsbecause maturing of the ceramic has been accomplished.

The important order of steps in the practice of our process is firstfiring and thereafter flattening of the encased heating element. Betweenthese two steps the initially straight sheathed element may be formed toany required or desired shape such as that shown in Figure 6 to fit thebottom of a cooking kettle or the like. If a strip heating element isdesired, the forming step just referred to can be omitted and of coursethe heating element may be formed U-shaped or any other desired shape,Figure 6 showing merely an example.

In Figure 4 we show an element flattened suitably for contact with acooking kettle as in electric stove elements and hot plates. In Figure 5we show another form of flattening suitable for the sheath to besoldered or brazed to the bottom of a cooking kettle, coffee pot, or thelike.

In actual tests we have found that a heating element flattened afterfiring as compared to the unflattened element shown in Figure 3 operatesat a higher over-all temperature which indicates that more heat is drawnfrom the resistance wire whereas the unflattened heating element has ahigher temperature in the resistance wire itself but conduction toceramic and the sheath is not as efficient and therefore the resistancewire has a shorter life as well as being less eflicient.

Due to firing causing some shrinkage of the core 12a, it is importantthat this shrinkage be taken up before the heating element is flattened.This is another reason that firing before flattening is desirable as iffiring were accomplished after flattening, there would be shrinkage ofthe core that would reduce the efliciency as distinguished fromincreasing it by flattening after firing. In order to insert the core12a into the sheath 20 initially the difference in diameters isnecessarily about .003 to .007. The firing operation is desirably doneat a relatively low temperature of about 1400 F. The flattening processthen reduces the internal area and the fired ceramic becomes hard anddense and fills all interstices, thus increasing heat conduction throughthe ceramic to the sheath to a maximum.

Aside from excellent pressure bonding between the ceramic and the insideWalls of the sheath for good heat transfer, the flattening processcompacts the insulation so tightly that it provides uniform distortionof the resistance coil and accordingly proportionate insulation spacingis provided throughout the area of the coil as illustrated in Figures 4and 5. The compressed ceramic fills any possible cracks that might bepresent because it has no place else to go and the area within thesheath is being reduced due to the flattening.

Our process permits the use of a relatively inexpensive extruded heatingelement to produce a highly eflicient finished heating element of thesheathed type without the expense heretofore necessary in connectionwith heating elements of the type having tamped powdered magnesium oxideas an insulator and without the difliculty experienced with those typesof heating elements in keeping the resistance wire centered during thetamping process. The extrusion process disclosed in the Smith patentsaccurately centers the heating element and after the core 12a is reducedby firing to a ceramic, any flattening of the sheathed element as wepropose causes proportionate deformation of the resistance coil whereasflattening while the core is still soft (before it is fired) wouldpermit the core to flow around the resistance wire and the resistancewire to retain its original circular shape instead of beingproportionally flattened as necessary to prevent reducing the thicknessof the insulation and to prevent the resistance wire from groundingagainst the sheath which of course cannot be tolerated. The sheath canthen be further formed and even portions thereof twisted if desiredwithout danger of the resistance coil shorting against the sheath.

Some variations in the steps of our process for producing the disclosedtype of heating element may be tolerated without departing from the realspirit and purpose of our invention, and it is our intention to cover byour claims any modifications or use of mechanical equivalents which maybe reasonably included within their scope.

We claim as our invention:

1. A method of producing a heating element compristo reduce the areawithin the sheath and crumble the ceramic, thereby tightly compressingthe ceramic around the resistance element and causing it to completelyfill said sheath.

2. In a method for producing a heating element, the steps of extrudingargillous material and a heating coil, saidextruded material beingessentially composed of a common clay of the type which, when fired,Will yield a ceramic which upon compressing crumblesinto a fine granularform capable of rearrangement of shape, sheathing the same in a metalsheath, heating said heating coil, argillous material and sheath totransform the argillous material to a ceramic, forming the resultingsheathed heating element to a desired shape substantially in one planeafter it has been heated, and flattening the same under high pressure tocrumble and tightly compress the ceramic in close contacting engagementwith the confining walls of the sheath.

3. A method of producing a heating element comprising the steps ofextruding an argillous core with a resistance element imbedded therein,said extruded core being essentially composed of a common clay of thetype which, when fired, will yield a ceramic which upon compressingcrumbles into a fine granular form capable of rearrangement of shape,encasing the same in a tubular metallic sheath, firing the encasedresistance element and core to about 1400 F. to reduce the core to aceramic, and flattening the sheathed heating element under a pressurebetween 2000 and 10,000 p. s. i. after it has been fired in order tocrumble and tightly compress the ceramic around the resistance elementand cause the same to completely fill said sheath.

4. A method of the character disclosed comprising the steps of extrudinga plastic insulating core and a heating element imbedded therein, saidextruded core being essentially composed of a common clay of the type,which, when fired, will yield a ceramic which upon compressing crumblesinto a fine granular form capable of rearrangement of shape, placingthem in a tubular metallic sheath, heating the sheathed heating elementand core to reduce the core to a ceramic, and flattening at least oneside of the sheathed heating element under high pressure to crumble andtightly compress the ceramic into close contacting engagement with saidmetallic sheath for maximum heat conduction from said heating element tosaid sheath.

References Cited in the file of this patent UNITED STATES PATENTS394,207 Paige Dec. 11, 1888 786,257 Beebe Apr. 4, 1905 1,528,388 SpeirsMar. 3, 1925 1,669,385 Wiegand May 8, 1928 1,987,915 Smith July 15, 19352,375,058 Wiegand May 1, 1945 2,475,756 Peulet July 12, 1945 2,403,022Reimers July 2, 1946

